Squamous cell carcinoma of the lung is typically preceded by a gradual and orderly sequence of recognizable cytomorphologic states of bronchial metaplasia and dysplasia. Such atypical changes, however, can be reversed by attenuation of smoking or by drug intervention; and therefore do not reliably predict the eventual development of frank neoplasia. Monoclonal antibody 17.13., a novel antibody raised against a human epidermoid carcinoma of the larynx, recognizes with high sensitivity and specificity an undefined cytoplasmic antigen(s) in normal human basal cells and in malignant human squamous cells from a variety of sites. Based on the finding that occasional sputum bronchial cells that morphologically are neither normal of frankly malignant are also MAb 17.13-reactive, we postulate that MAb 17.13. may recognize an early marker of neoplastic commitment among superficial layers of bronchial epithelium including the cells exfoliated into sputum. This application proposes (a) a survey of the prevalence of MAb 17.13-reactivity in the bronchial epithelium of 150 former asbestos workers at high risk for lung cancer; and (b) the modulation of that reactivity by a six-month course of the retinoid etretinate in a single are Phase II trial. Results of this pilot study in methods-development will allow a sound assessment of the feasability of this model for the future testing of putative chemoprevention agents among cohorts at high risk for lung cancer.